The present invention relates to a raw material for chromium targets with a high oxygen content and the method of producing thereof, and relates more particularly to a technique which provides a raw material for chromium targets capable of effectively preventing dust generation in forming films by sputtering.
The sputtering method is widely applied to the production of thin film materials in the fields of electronics, optics, precision instruments, cutting tools, etc. This sputtering method comprises colliding argon or other ions against a target obtained by shaping and processing a raw material as a vapor deposition source, and depositing the particles ejected from the target on materials to be vapor-coated such as substrates, wafers, etc., thereby forming a thin film.
In recent years, the use of metallic chromium as a raw material for vapor deposition targets produced to apply the base film, etc. of magnetic recording media such as Co-Ni, etc. has been increasing because of the excellent optical properties, electrical properties and corrosion resistance of base films produced with chromium targets.
Under these circumstances various methods of processing metallic chromium by casting or powder metallurgy are being examined.
As a method of producing metallic chromium by powder metallurgy, the method of pressing high purity metallic chromium powder by the means of hot isostatic pressing (hereinafter referred to as "HIP") etc. are known.
In the Japanese Patent Publication No. 48868/1992, a chromium target material for forming the base film of a magnetic recording medium comprising less than 1,000 ppm of O, less than 300 ppm of N, and the substantial balance of chromium is disclosed, and also in this patent publication disclosure, it is described that a high purity chromium target produced by the powder metallurgy is preferably used.
This prior art is characterized by taking into consideration O and N as impurity elements and controlling the amount of O and N under certain limits, thereby obtaining a high coercive force and a high sputtering speed.
In order to improve the coercive force and the square ratio, as disclosed in the prior art, less impurities are desirable, and therefore it is desired that the contents of the impurities such as O, N, etc. should be minimal.
In order to obtain a magnetic recording medium with a high recording density, less impurity elements are not always desirable, and on the other hand, there are some cases where a higher oxygen content can contribute to an improved recording density.
Under these circumstances, there is a increasing demand for a chromium target with a relatively high oxygen content and for its raw material.
In general, however, the solid solubility limit of O in Cr is extremely low, and it is well known that most of the oxygen is present as chromium oxide (Cr.sub.2 O.sub.3) in the form of inclusions by adding for example, some thousands of ppm of oxygen to metallic chromium and by having a heat history with temperatures above 800.degree. C.
Therefore, if such an inclusion is present randomly in the target, electrification of the inclusion or a local abnormal discharge occurs during film-forming by sputtering, and consequently the inclusions are destroyed, or the problem of the so called "dust ejection" phenomenon arises wherein the inclusions are separated from the target and float in the film-forming chamber.
Such dust ejection is most undesirable for improving the recording density of the magnetic material, because the floating dusts are taken in the film, or they adhere to the film, thereby forming projections. Especially, with the increasing recording density of the magnetic medium, there is a tendency toward a higher possibility of a fatal fault, the so-called head crash, wherein the projections on the magnetic film crash against the magnetic head when the flying height of the head is lower than 1 .mu.m.
It is therefore the object of the present invention to improve the oxygen containing form of the metallic chromium used for chromium targets, and specifically to obtain a structure incongruous with dust generation from the target in forming a thin film of the target material on the substrate of the recording medium, and to provide a raw material (sintered compact or powder) for chromium targets capable of improving the recording density.
As a result of having concentrated their energies to accomplish the above object, the inventors invented the present invention whose outline is as follows:
Namely, the present invention relates to a raw material for high oxygen chromium targets comprising chromium oxide crystalline particles dispersed almost uniformly in metallic chromium with at least a portion of the surface of all the crystals constituting the oxide crystalline particle being in contact with the metallic chromium.
According to the present invention, the chromium oxide crystalline particles have a particles size of 0.1 to 100 .mu.m and preferably of 0.1 to 20 .mu.m.
The method of producing a raw material for high oxygen chromium targets according to the present invention comprising chromium oxide crystalline particles dispersed in metallic chromium is characterized by heat-treating metallic chromium containing a solid solution of oxygen or oxide particles as a starting material, and by precipitating or crystal-growing its oxygen or oxide particles as chromium oxide crystalline particles having a particle size of 0.1 to 100 .mu.m, thereby obtaining a raw material for chromium targets.
Another method of producing a raw material for high oxygen chromium targets according to the present invention comprising chromium oxide crystalline particles dispersed in metallic chromium is characterized by mixing metallic chromium powder containing a solid solution of oxygen or oxide particles with chromium oxide powder, then heat-treating the mixed powder, and by precipitating and crystal-growing the oxygen, oxide particles and oxide powder as chromium oxide crystalline particles having a particle size of 0.1 to 100 .mu.m, thereby obtaining a raw material for chromium targets.
Heat-treating can be performed during or after the process in which metallic chromium or a mixed powder of metallic chromium powder and chromium oxide powder is compacted into a predetermined form by means of a hot isostatic press.